According to AS (h/t Sport Witness), the quartet are all monitoring the keeper's situation ahead of the summer, though the Slovenian has a €100 million (£88 million) buyout clause as part of his deal, which runs until 2021, and it would appear that fee would need to be met to prise him away from Los Rojiblancos.
The 25-year-old has had yet another outstanding season for Atleti, as the club illustrated through its official Twitter account:
Oblak has impressed throughout his four-year spell in the Spanish capital and is among the best in his position in the world.
Bleacher Report's Sam Tighe would even place him ahead of Manchester United star David De Gea in that regard:
Considering his ability, in today's market even €100 million may not be such an unreasonable price to pay for him, although it remains a significant sum.
Arsenal, in particular, should jump at the chance to sign him. The Gunners signed Chelsea legend Petr Cech in 2015, but the 35-year-old has been in visible decline for much of his time at the Emirates Stadium.
FourFourTwo revealed further insight into the stopper's struggles:
Liverpool would have more of a dilemma than the Gunners. While Oblak would undoubtedly be an upgrade on Loris Karius, the German has been an impressive figure between the sticks since Reds boss Jurgen Klopp made him Liverpool's No. 1 earlier in the season.
A consistent run of games has allowed Karius to improve, with football writer Leanne Prescott praising the way he has done so:
Oblak could also make a strong replacement for Gianluigi Buffon at Juventus if he retires this summer, though the Bianconeri have been lining up Wojciech Szczesny to fill his considerable gloves, so spending big on another goalkeeper seems unlikely, even if he'd be a stronger option.
As for PSG, a goalkeeper of Oblak's stature could play a big role in helping them become a force on the European stage, and of the four interested parties, they would perhaps be best-equipped to fund a pursuit of him, depending on any financial-fair-play concerns brought on by the acquisitions of Neymar and Kylian Mbappe.
Prising him away will still be difficult even if his buyout clause is met, though, so it's far from guaranteed he'll leave even in the event one of his suitors is prepared to take the plunge.